(a) Field of the Invention
The present invention provides a method of removing phosphorus and/or nitrogen. More specifically, the present invention provides an efficient and economical method of selectively or simultaneously removing phosphorus and/or nitrogen from wastewater containing phosphorus and/or nitrogen through struvite crystallization.
(b) Description of the Related Art
There are many existing biological and physical-chemical methods for removing phosphorus and nitrogen and these methods have drawbacks such as difficulties in maintaining the removal efficiency while being costly as well.
Recently, the research of simultaneous removal of nitrogen and phosphorus through production of struvite (magnesium ammonium phosphate, MAP) has been actively pursued. Pure MAP is composed of magnesium ion, ammonium nitrogen, and phosphate in a mole ratio of 1:1:1, reciprocally. However, existing studies show that there are many side reactions and such side reactions cause the mole ratio exhibiting a maximum nitrogen elimination to vary as the likes of 3:1:1.5. The reaction formula is:Mg2+NH4+PO4'−6H2O→MgNH4PO46(H2O)
Additionally, struvite analogs such as Calcium Ammonium Phosphate, in which magnesium ion is substituted by calcium ion, may be produced. As a matter of fact, a knowledgeable amount of phosphorus and/or nitrogen is removed in the form of struvite analogs as well as struvite, but recognition of struvite analogs is absent in many struvite related researches. In this invention, the category ‘struvite’ includes struvite analogs along with struvite.
Removal of phosphorus and nitrogen from wastewater by the form of struvite and struvite analogs can effectively eliminate phosphorus and nitrogen simultaneously. It also has the advantage of producing struvite that can be sold as a slow-release fertilizer.
The term ‘slow-release fertilizer’ refers to a fertilizer that is non-toxic and has a lasting effect without being easily oxidized over a long period of usage.
The researches for various struvite crystallization methods have been performed internationally. These include:
S. Regy et al., Phosphate recovery by struvite precipitation in a stirred reactor (March to December 2001)
Ueno and Fujii, Three years experience on operating and selling recovered struvite from full scale plant (2001)
E. V. Munch et al., Making a business from struvite crystallization for wastewater treatment: turning waste into gold (2001)
S. A. Parsons et al., Assessing the potential for struvite recovery at sewage treatment works (2001)
Trentelman, U.S. Pat. No. 4,389,317
Tsunekawa et al., Abstracts of Japanese Patent No. 11-267665
Many methods to remove phosphorus and/or nitrogen in struvite form have been developed and most of them need addition of an alkaline substance to increase the pH.
Many researchers have reported that the appropriate pH for MAP formation is from 8 to 10. The optimum pH is reported as 9.0 to 9.2. However, most of the developed MAP producing technologies use alkaline substances to increase the pH, but fail to achieve a pH higher than that of 9, due to high expenses of NaOH addition. On top of that, maintaining a fluidized bed is required in order to increase the size of crystals.